1. Field of the Invention
The present invention relates generally to silicon nitride insulator layers and silicon nitride insulator structures within integrated circuits. More particularly, the present invention relates to automated methods for monitoring and controlling the aqueous ortho-phosphoric acid (H3PO4) solution etch rate of silicon nitride insulator layers and silicon nitride insulator structures within integrated circuits.
2. Description of the Related Art
Integrated circuits are formed from semiconductor substrates within and upon whose surfaces are formed resistors, transistors, diodes and other electrical circuit elements. The electrical circuit elements are connected internally and externally to the semiconductor substrate upon which they are formed through patterned conductor layers which are separated by insulator layers.
In addition to forming insulator layers within integrated circuits from the common insulator material silicon oxide, insulator layers within integrated circuits may also be formed from other insulator materials which provide novel and/or desirable insulator layer properties, such as, but not limited to, insulator layer dielectric properties and insulator layer etch resistance properties. A common other insulator material often employed in forming such insulator layers within integrated circuits is silicon nitride. Insulator layers formed from silicon nitride within integrated circuits are often employed in forming etch stop insulator layers beneath and/or above silicon oxide insulator layers within integrated circuits, as well as in forming capacitor insulator layers within capacitors formed within integrated circuits.
When employing silicon nitride insulator layers within integrated circuits, it is common in the art that those silicon nitride insulator layers will be patterned or stripped through a wet chemical etch method employing an aqueous ortho-phosphoric acid (H3PO4) solution at an elevated temperature, typically an elevated refluxing temperature at about 165 degrees centigrade. The chemical reaction by which silicon nitride is etched in an aqueous ortho-phosphoric acid (H3PO4) solution at elevated temperature is illustrated by equation (1). EQU 3 Si3N4+27 H2O+4 H3PO4.fwdarw.4 (NH4)3PO4+9 H2SiO3 (1)
As seen by equation (1), when etching silicon nitride with an aqueous ortho-phosphoric acid (H3PO4) solution there is consumed in addition to a moderate stoichiometric excess of ortho-phosphoric acid (H3PO4) in comparison with silicon nitride a substantial stoichiometric excess of water in comparison with both silicon nitride and ortho-phosphoric acid (H3PO4).
As water is consumed during the etching of a silicon nitride insulator layer or a silicon nitride insulator structure in an aqueous orth-phosphoric acid (H3PO4) solution, a secondary series of reactions occurs wherein the ortho-phosphoric acid (H3PO4) within the aqueous ortho-phosphoric acid (H3PO4) solution is dehydrated through an intermediate pyro-phosphoric acid (H4P2O7) and an intermediate meta-phosphoric acid (HPO3) to yield phosphorus pentoxide, as illustrated by the series of dehydration reactions shown by equation (2) to equation (4). EQU 2 H3PO4.fwdarw.H4P2O7+H2O (2) EQU H4P2O7.fwdarw.2 HPO3+H2O (3) EQU 2 HPO3.fwdarw.P2O5+H20 (4)
Through the series of dehydration reactions shown by equation (2) to equation (4), the concentration of ortho-phosphoric acid (H3PO4) within the dehydrated aqueous ortho-phosphoric acid (H3PO4) solution may typically be sufficiently lowered such that the etch rate of a silicon nitride insulator layer or silicon nitride insulator structure in contact with the dehydrated aqueous ortho-phosphoric acid (H3PO4) solution is also significantly decreased, often with an accompanying compromise of etch selectivity of the silicon nitride insulator layer or the silicon nitride insulator structure with respect to other layers adjoining the silicon nitride insulator layer or silicon nitride insulator structure, such as, but not limited to, silicon oxide insulator layers and silicon substrate layers.
An additional unfortunate consequence of the series of dehydration reactions shown by equation (2) to equation (4) is that phosphorus pentoxide is a solid material with limited solubility within dehydrated aqueous ortho-phosphoric acid (H3PO4) solutions at temperatures at which those dehydrated aqueous ortho-phosphoric acid (H3PO4) solutions may be employed in etching silicon nitride insulator layers or silicon nitride insulator structures. At sufficiently high concentration, the solid phosphorus pentoxide often precipitates from dehydrated aqueous ortho-phosphoric acid (H3PO4) solutions, thus yielding a particulate contaminant residue upon the surfaces of semiconductor substrates from whose surfaces it is desired to etch silicon nitride insulator layers or silicon nitride insulator structures through contact with those dehydrated ortho-phosphoric acid (H3PO4) solutions.
Although it is possible to re-hydrate through manual additions of water dehydrated aqueous ortho-phosphoric acid (H3PO4) solutions employed in etching silicon nitride insulator layers and silicon nitride insulator structures within integrated circuits, such manual additions of water will: (1) if sporadically made yield re-hydrated aqueous ortho-phosphoric acid (H3PO4) solutions with undesirably variable etch rates for silicon nitride insulator layers and silicon nitride insulator structures, and (2) if excessively made yield re-hydrated aqueous ortho-phosphoric acid (H3PO4) solutions with undesirably low etch rates for silicon nitride insulator layers and silicon nitride insulator structures.
From the foregoing, it is therefore desirable within the art of etching silicon nitride insulator layers and silicon nitride insulator structures within integrated circuits with aqueous ortho-phosphoric acid (H3PO4) solutions to provide a method by which the ortho-phosphoric acid (H3PO4) concentration within the aqueous ortho-phosphoric acid (H3PO4) solution may be constantly monitored and controlled. It is towards that goal that the present invention is directed.
Methods through which solutions of materials employed in manufacturing integrated circuits and microelectronics products may controlled to uniformly and reproducibly perform a manufacturing function within those integrated circuits and microelectronic products are known in the arts of integrated circuit manufacture and microelectronics products manufacture. For example, Morris, in U.S. Pat. No. 4,092,211 discloses a method for controlling within a boiling aqueous ortho-phosphoric acid (H3PO4) solution the etch rate of a silicon oxide insulator layer which is employed in masking a silicon nitride insulator layer. The method employs the deliberate addition of a silicate material to the boiling aqueous ortho-phosphoric acid (H3PO4) solution. In addition, Bell et al., in U.S. Pat. No. 5,332,145 disclose a method for continuously monitoring and controlling the compositions of low-solids soldering fluxes that employ a solvent with a specific gravity closely matched to the specific gravity of the flux composition. The method employs an ultra-violet absorption detector to monitor and control the solvent content within the flux composition.
Desirable in the art are analogous methods through which the etch rates of silicon nitride insulator layers and silicon nitride insulator structures in aqueous ortho-phosphoric acid (H3PO4) solutions may be monitored and controlled. Particularly desirable are analogous methods through which the etch rates of silicon nitride insulator layers and silicon nitride insulator structures within aqueous ortho-phosphoric acid (H3PO4) solutions may be continuously monitored and controlled.